The Reagan administration broke 44 months of silence yesterday to call suddenly for ratification of the international treaty against genocide, which has been bottled up in the U.S. Senate for 35 years.

Administration sources said the announcement, which had been in the works for several weeks, was made hastily after the White House learned Tuesday that Democratic presidential nominee Walter F. Mondale planned to make an issue of the genocide treaty today.

Both Reagan and Mondale are scheduled to speak today to the national convention of B'nai B'rith, the largest Jewish service organization in this country. B'nai B'rith has been one of the foremost backers of the treaty.

Reagan's decision to join all post-World War II presidents except Dwight D. Eisenhower in supporting the anti-genocide convention was announced at the State Department as the president completed his opening swing of the fall political campaign.

Alexander M. Haig Jr. supported ratification of the treaty during the hearings on his nomination to be Reagan's first secretary of state in January 1981, saying it would "unquestionably be helpful" to U.S. diplomacy. But the administration took no position on the question until yesterday.

The Convention on the Prevention and Punishment of the Crime of Genocide, as it is formally known, was prompted by the deliberate annihilation of Jews in Nazi Germany during World War II. The United States played an important role in drafting the convention, which was signed by the United States on Dec. 11, 1948, and was submitted to the Senate for ratification by President Truman on June 16, 1949.

Despite the support of Truman and Presidents Kennedy, Johnson, Nixon, Ford and Carter and the ratification by 96 countries, the treaty has languished in the Senate because of opposition by some conservatives and members of organizations such as the Liberty Lobby and John Birch Society.

The convention was also opposed for many years by the American Bar Association. The ABA reversed its position in February 1976 to become one of the treaty's strongest supporters.

The treaty, which is mostly symbolic, declares genocide to be an international crime to be prevented and punished. Genocide is defined as the intentional destruction of any national, ethnic, racial or religious group, in whole or in part, by killing its members, causing them serious physical or mental harm, imposing conditions of life intended to bring about their physical destruction, imposing measures intended to prevent births or transferring children from one group to another.

State Department officials said legislation will be prepared to make genocide a crime under U.S. law, but added that most of the elements are already illegal under civil rights laws and other U.S. statutes.

At a briefing for reporters, the State Department officials insisted that the recent completion of "a very thorough review which looked back at the 35-year history" of the convention, rather than the political contest, dictated the timing of the announcement. "I would reject the premise that the announcement is connected to any particular speech or political event," said a State Department officer who briefed reporters on condition that he not be identified by name.

Administration sources who also declined to be named said Gov. George Deukmejian of California, a longtime political ally of Reagan, spoke recently to the president about the genocide convention, emphasizing the backing for it in the Armenian community as a symbolic reproach to the Turks for the massacres of Armenians in the late 19th and early 20th centuries. These sources said Reagan had been impressed by the plea and expressed interest in taking action.

Sen. Charles H. Percy (R-Ill.), chairman of the Senate Foreign Relations Committee and long a backer of the treaty, praised Reagan's action and said he would hold new hearings and obtain a committee vote on the measure within two weeks.

An aide to Senate Majority Leader Howard H. Baker Jr. (R-Tenn.) said, however, that the administration action was a complete surprise to Baker until two hours before the announcement.

The Liberty Lobby, which was the only organization to appear in opposition in the 1981 Senate Foreign Relations Committee hearings, the last ones held, issued a statement criticizing Reagan for "bowing to the pressures of an election year."